Selective Flocculation of Fine Particle Specularite Using Carboxymethyl Starch: Role of Carbon Chain Length and Underlying Mechanisms

This study investigates the selective flocculation and high-gradient magnetic separation of fine-grained specularite (−12.5 μm) and quartz, utilizing two representative carboxymethyl starch (CMS) flocculants with distinct carbon chain length: short carbon chain carboxymethyl corn starch (CMCS) and long carbon chain carboxymethyl cassava starch (CMTS). The results demonstrate that CMS significantly enhances the magnetic separation recovery of specularite. Under optimal conditions, CMCS with a degree of substitution (DS) of 0.60 and CMTS with a DS of 0.25 increased the magnetic recovery of specularite from 72 % to 93 % and 92 %, respectively. Furthermore, the iron grade of the magnetic concentrate from the binary mixture (specularite and quartz) was elevated by 4 % and 6 %, while iron recovery rates improved by 17 % and 24 %, respectively. Zeta potential measurements indicate that CMS predominantly adsorbs onto the surface of specularite, with the short carbon chain CMCS-60 exhibiting stronger selectivity than the long carbon chain CMTS-25. Infrared spectroscopy (FTIR) further suggest that the carboxymethyl groups of CMS undergo strong chemical adsorption with the Fe sites on the specularite surface but hardly adsorbs on quartz. Polarized light microscopy and sedimentation tests reveal that CMS selectively adsorbs onto the specularite surface, facilitating aggregation into larger flocs, thereby improving the magnetic separation recovery of specularite. Notably, CMTS-25, the long carbon chain flocculant, promotes more efficient aggregation of specularite. However, neither CMS interacts with quartz, and the growth of specularite flocs in the binary system results in the entrapment of some quartz particles, thereby diminishing the separation efficiency. These findings provide new insights into the role of CMS in sustainable mineral processing, highlighting its potential for improving fine iron ore beneficiation while minimizing environmental impact.